Previously, a structural member for a car, such as a door-impact-protection beam which is a door stiffening member, has been widely used for securing the safety of a driver in the case of a lateral collision of a car. As a door-impact-protection beam, an ultra-high-tensile steel pipe has been adopted for securing collision safety and realizing low fuel consumption by weight reduction. As a material for such a steel pipe, a material having a tensile strength of not less than 1,400 MPa and a cross section of a perfect circle has been conventionally used.
It is preferable to further reduce fuel consumption by reducing the weight of car components and the like, and thus a lighter door-impact-protection beam in weight is required. Therefore, a door-impact-protection beam having the thinnest possible wall thickness may be desired. However, as a door-impact-protection beam can be used for securing safety at the time of collision and is required to secure flexural strength and absorbed energy performance more than a certain level, the weight reduction of a conventional door-impact-protection beam having a cross section of a perfect circle may have reached a limit.
One of the objects of the present invention is to provide a structural member for a car, such as a door-impact-protection beam which is a door stiffening member, a pillar stiffening member, a bumper stiffening member, etc., which structural member being capable of solving the above-mentioned conventional problems, having flexural strength and absorbed energy performance equal to or exceeding a conventional steel pipe with a circular cross section when a car collision (for example, a head-on collision, a lateral collision, etc.), particularly a lateral collision, occurs, and moreover being capable of attain weight reduction further than a conventional structural member and a car body fabricated using said structural member for a car.